Page 16 - Theory and Design of Air Cushion Craft

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Introduction to hovercraft

1.1 Hovercraft beginnings

Transport is driven by speed. Since the 1970s, with the price of fuel becoming an
important component of operating costs, transport efficiency has become a significant
factor guiding concept development. During the last century, the service speed of
many transport concepts has dramatically increased, taking advantage of the rapid
development of internal combustion engines. Aeroplane flying speed has increased by
a factor of 10, and the automobile by a factor of three. In contrast, the highest com-
mercial ship speeds have increased by less than a factor of two, to a service speed of
about 40 knots.
Some planing craft and fast naval vessels reached this speed in the 1920s. They were
able to do this because payload was not a key requirement, so that most of the carry-
ing capacity could be devoted to power plant and fuel. Hydrodynamic resistance was
the prime factor limiting their performance. A displacement ship moving at high
speed through the water causes wavemaking drag in proportion to the square of its
speed. This limits the maximum speed for which a ship may be designed, due to prac-
tical limitations for installed power. It is possible, however, to design ship forms using
the surface planing principle to reduce wavemaking at higher speeds. Many planing
boat designs have been built, though the power required for high speed has limited
their size. Their application has mostly been for fast pleasure and racing craft, and for
military vessels such as fast patrol boats.
Planing vessels demonstrated the potential for increased speed, but slamming
caused by wave encounter in a seaway still created problems for crews, passengers and
the vessels themselves, due to high vertical accelerations. Two possibilities to avoid
slamming are either to isolate the hull from contact with the water surface, or sub-
merge it as completely as possible under the water to reduce surface wave induced
drag. Hydrofoils, air lubricated craft, amphibious hovercraft (ACV), surface effect
ships (SES) and wing in ground effect machines (WIG and PARWIG) arose from the
first idea, while the latter concept produced the small waterplane thin hull vessel
(SWATH) and, more recently, thin water plane area high speed catamarans. Fig. 1.1
shows a classification of high speed marine vehicle types.
ACV and SES - the subject of this book - developed from the idea to design a craft
which is supported by a pressurized air 'cushion'. By this means the hard structure is